The present invention relates to telecommunications exchanges and in particular to ring trip detection ciruitry for such exchanges.
To alert telephone customers to a telephone call telephones are equipped with a calling device (a bell or other acoustic transducer) which is responsive to alternating current electric signals applied to the customers line at the telephone exchange. At the same time the calling customer receives tones indicating that the call has been switched through.
When the called customer answers the telephone it is necessary for the calling signal to be removed from that customers line, preferably near instantaneously, to avoid an intense sound being emitted from the telephone transducer. The calling tone indication must be removed from the calling customers line to indicate that the called customer has answered.
Removal of the calling signal and tone from the lines is known as `Ring Trip` and ring trip detection is an inherently difficult function to perform. In some exchange installations ring trip detection is accomplished in software program of a computer control system and is limited accordingly by sampling rate that is the rate at which each line is considered by the computer to determine whether ring trip has occurred. Thus delay in detection of ring trip is possible.
In other exchange installations hardware on a per-line basis is provided which is a more costly solution. Such systems use analog detectors via analog filters for example.
It is an object of the present invention to provide a digital ring trip detector which is less susceptible to time delays resulting from sampling rate and which is less costly than the analog line equipment mentioned above.